The present invention relates to fuel management for pressurized water nuclear reactors, and in particular to the arrangement of nuclear fuel assemblies within a reactor core.
Modern commercial nuclear power reactors are fueled with uranium having a slightly enriched U-235 content, which necessitates that portions of the core be periodically removed and replaced with fresher fuel. The plan of replacement and arrangement of fuel during the life of the reactor, known as in-core fuel management, is a major design consideration, having both safety and economic consequences. In a typical pressurized water nuclear power reactor (PWR), the initial core loading consists of three approximately equal sized batches of fuel assemblies having different enrichments. In conventional terminology, batch A has the lowest enrichment, batch B a higher enrichment, and batch C the highest enrichment. At the end of the first cycle, typically one year in length, batch A is removed from the reactor, batches B and C are rearranged, and a feed batch D of fresh fuel is placed in the reactor. This procedure is typical of three batch incore fuel management wherein an entire batch of fuel is removed and replaced with the same number of feed fuel assemblies every year for the life of the plant. It is usually desirable to achieve an equilibrium in-core fuel management scheme as early as possible in the plant lifetime, such that the feed assemblies will always have the same enrichment and will be placed in the same locations as the previous feed assemblies, and the once-burned and twice-burned assemblies that remain in the core will be shuffled to identical locations occupied by the previously once and twice-burned assemblies.
Having introduced the nature of the art to which the invention pertains, and before proceeding to a more detailed description of the background of the invention, a review of the terminology commonly used in the art of nuclear reactor fuel management will be presented with a view towards defining the terms for specific use herein.
A fuel assembly is a square array of fuel rods connected at their ends by end fittings to form a unit that is insertable and removable from the core. Other structure that remains fixed with respect to the fuel rods and end fittings during a particular cycle is also considered part of the fuel assembly. The fuel lattice within the assembly is the array of fuel rod locations of the assembly, excluding water holes. Water holes are locations in the fuel assembly where fuel rods are intentionally omitted, usually in order to provide space for instrumentation or for a control rod guide tube. These tubes are part of the structural support of the assembly and provide guides wherein control rods may be reciprocated. Fixed burnable poison shims are solid material in the fuel assembly containing parasitic neutron absorbing poison having a concentration which permits most or all of the poison to be consumed during one or more cycles in the reactor. The enrichment of the fuel rods relates to the fissile isotope content at the time of first introduction into the reactor core, i.e., when it is fresh, or feed, fuel.
A batch is a group of fuel assemblies that are placed into, and then permanently removed from, the core together. A lot is a group of fuel assemblies that are placed into the core at the same time, but which may or may not be permanently removed at the same time. A cycle is the time during which the arrangement of normally stationary fuel in the reactor core is unchanged, usually beginning with the placement of a feed batch or lot of fresh fuel into the core, and ending with the removal of highly burned assemblies. For the purposes relevant hereto, a cycle requires that the fissile material in the fuel actually produce power and experience partial depletion. Typical cycles range from 10 to 15 months in duration. The number of burns an individual fuel assembly or a lot of fuel has experienced is the number of cycles it has been in the reactor core.
A checkerboard is a pattern, superimposed on a grid region of adjacent parallel rows and columns of uniformly spaced squares, that is similar to the red and black color pattern that appears on the checkers game board. A checkerboard is characterized in that a line drawn through the diagonal of a single red square will, if extended in either direction throughout the region, intersect only red squares, and similarly for the black squares. In the present context, checkerboarding fuel assemblies in the reactor core means that one type of types of assemblies correspond to the red component squares on the game board, and other types of assemblies correspond to the black component squares on the board. The core periphery consists of the fuel assembly locations in the core where more than a mere corner of a fuel assembly borders on the neutron reflector at the outer boundary of the core.
It is a primary purpose of in-core fuel management to minimize the amount of U-235 or other fissile material required for a given energy output during a given cycle. This can be appreciated by the rule of thumb that for every 0.1 effective weight percent (wt%) increase in required core average enrichment, the increased cost of fuel for that cycle is over 2 million dollars. Typical equilibrium cycle core average enrichments are about 3.3 wt% U-235. It can also be appreciated that the greatest savings in overall fuel costs will be achieved by minimizing the feed enrichment required for an equilibrium fuel management scheme.
The major constraint on the flexibility of in-core fuel management is imposed by very strict power distribution limitations required by safety considerations. For example, the predicted ratio of the powers produced in the hottest fuel rod to the core average fuel rod is typically not permitted to exceed 1.40. This imposes correlative requirements on the ratio of power produced in a fuel assembly to the core average assembly power, and on the maximum rod power within an assembly to the average power in the assembly containing that rod. In modern commercial PWR's, fixed burnable poison shims are frequently located in selected assemblies to control the power distribution. These shims are strongly absorbent when the assembly is first placed in the core, and become weaker the longer they are exposed to the operating core environment. Although the shims are useful for controlling the power distribution and other core characteristics such as the moderator temperature coefficient, the presence of residual shim poison at the end of a cycle presents an inherent reactivity penalty, and requires a greater initial U-235 enrichment (and cost) at the beginning of each cycle in order to overcome the parasitic neutron absorbing effect of the residual.
The use of shims as a power shaping means have traditionally been directed primarily to controlling the power distribution within and between assemblies, but the use of significant numbers of shims will also affect the gross power shape in the reactor. This has economic consequences in that a power shape that is peaked radially toward the center of the core will be more efficient in conserving neutrons within the reactor so that they may produce additional fissions, than a power shape that is peaked near the core periphery, where neutrons will leak out of the reactor and never return. Thus, for the same core average initial enrichment (and assuming zero end of cycle shim residual), a longer cycle can be achieved when the power shape near end of cycle is centrally peaked than when it is more uniform or peripherally peaked.
FIG. 1 symbolically shows two of the most common prior art fuel management techniques implemented in a core having 241 fuel assembly locations. Each is a three batch second cycle scheme for achieving the same power level and cycle length, but the arrangement of the fuel types is characteristic of the respective schemes in other cycles. The highly reactive feed fuel (D) is shown as crosshatched squares 10, 10', the less reactive once-burned (C) fuel as open squares 12, and the least reactive once-burned (B) fuel as crossed squares 14. Note that in a first cycle all batches A, B, and C, would be fresh, but the different enrichments could be represented by the three symbols, and in cycles after the second the crossed squares 14 would represent twice-burned fuel.
In order to facilitate a later description of cycle-independent fuel management, fuel loadings will be designated by their relative lots. Thus, lot L is the feed or fresh lot 10, 10', L-1 the previously loaded fresh fuel 12, L-2 the next previously loaded fresh fuel 14, etc., except that in the first cycle L, L-1, and L-2 correspond to the customary C, B, and A lots, respectively, and in second cycle L, L-1, L-2, L-3 correspond to D, C, B, and A, respectively. In equilibrium cycles, the numerical portion N of the L-N designation can be thought of as the number of cycles the lot has previously resided in the core, i.e. the number of burns it has experienced.
The upper left quadrant (a) of FIG. 1 shows what is commonly referred to as the Out-In (OI) prior art fuel management scheme. This is characterized by unshimmed feed (L) fuel 10 placed at the core periphery to the extent possible. Any feed assemblies that are left over are located towards the periphery and surrounded to the extent possible by twice-burned (L-2) fuel 14. In the next cycle, the feed fuel will have become once-burned (L-1) fuel 12, and as shown in (a), the once-burned fuel 12 is concentrated in the core center. In general, the OI scheme has an inner region of once-burned fuel 12, a peripheral region of feed fuel 10, and an intermediate region of primarily twice-burned 14 mixed with some feed 10 and once-burned 12 fuel. It is noted that this concept for arranging fuel has also been used in first cycle designs.
The OI fuel management has the advantage of providing a relatively flat cosine gross core radial power shape, which helps avoid excessive local peaking. But as the fuel is depleted during the cycle, the gross power tends to shift towards the core periphery where the fresh fuel is located. The relatively high peripheral power, however, produces a high neutron leakage, especially at end of cycle (EOC) when the interior of the core has been depleted and the exterior is still relatively highly reactive.
A prior art attempt to improve the neutron economy and reduce the required enrichment of the OI scheme is shown in (b) of FIG. 1, which will be referred to as the In-Out-In (IOI) scheme. In this scheme, all feed (L) assemblies 10' contain burnable poison shims (represented by circles) and are placed towards the center of the core in a checkerboard pattern that alternates components of L assemblies 10' with components of twice-burned (L-2) assemblies 14. The L component is violated near the core center (assembly locations 50, 58) in order to accommodate the well known tendency of the power distribution to peak in this area. All once-burned (L-1) fuel 12 is placed a far as possible towards the core periphery. Thus, the IOI scheme is characterized by an inner checkerboard of feed fuel and twice-burned fuel and an outer region of once-burned fuel. The IOI scheme as practiced in the prior art requires that the shims in the feed fuel assemblies be removed after one cycle. The IOI scheme thus places fresh shimmed fuel in the center region, then removes the shims at the end of the cycle so that at the beginning of the next cycle the once-burned fuel contains no shims. Since the twice-burned fuel was previously a once-burned fuel, it also contains no shims.
The major advantage of the IOI scheme is the low neutron leakage from the core periphery resulting from the tendency of the power distribution to remain centrally peaked throughout the burnup cycle. In addition to permitting a lower initial enrichment for the same energy extraction, the centrally peaked power distribution produces a lower radiation exposure to the reactor internals and vessels surrounding the core, and has other advantages related to the stability of the power distribution.
The prior art requirement in the IOI scheme that the shims be removed at the end of the first cycle of exposure of the feed assemblies has an inherent disadvantage which limits the flexibility of fuel management. The purpose of shimming the feed fuel is to control the power shape so that the fresh fuel in the central region of the core will not produce power too great in relation to the core average power. This control requires such a high initial concentration of poison material in the shims that the poison does not burn out by EOC and therefore a significant parasitic effect remains. Nevertheless, the advantage of the lower neutron leakage from the periphery is greater than the disadavantage at end of cycle of having a significant posion shim residual. By removing these shims prior to the next cycle, the parasitic shim effect is not carried over into the next cycle. In order for the shims to be easily removed, however, they are placed in guide tubes normally reserved for control rods rather than being permanently integrated in the fuel lattice. This precludes the placement of feed assemblies under control rods in the IOI scheme, and thus eliminates one-third or more core locations from use with feed assemblies. Such loss of fuel placement flexibility can be particularly restrictive if the energy extraction or cycle length is to be varied from equilibrium IOI values. In order to best accommodate a non-equilibrium cycle or to optimize the return to equilibrium, fresh fuel might well be ideally placed in some of these locations yet cannot be without abandoning the prior art IOI scheme. In addition, the limited flexibility of the IOI scheme is even more evident if the scheme is used in cores employing more advanced control rod designs wherein up to twelve control rod fingers are insertable into five adjacent assemblies by the action of a single drive mechanism. These control rods permit greater reactivity control and offer other significant advantages, yet are generally impossible to use in cores having many of the control rod guide tubes occupied for other purposes, as in the IOI scheme.
If the prior art IOI scheme is modified so that the shims are left in each feed assembly at the end of cycle C and not removed when the assembly is shuffled into a once-burned location on the core periphery in cycle C+1, the residual shim absorption near the periphery will tend to accentuate the power near the core center, requiring that the cycle C+1 feed batch have even stronger shims to control the power peak at the beginning of cycle C+1. The fresh fuel will than have an even greater shim residual at the end of cycle C+1 and, when this fuel is placed on the periphery in cycle C+2 the peripheral power will be even further depressed requiring even stronger initial shim loadings on the next batch of feed fuel. The end of cycle residual thus would become so large as to dissipate the advantage in the IOI scheme of low neutron leakage.